Disentangling the phase sequence and correlated critical properties in Bi_0.7La_0.3FeO₃: a combined neutron diffraction and Raman scattering study.

This work addresses the study of the high temperature phase sequence of Bi_0.7La_0.3FeO₃ by undertaking temperature dependent high resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. The analysis revealed that Bi_0.7La_0.3FeO₃ exhibits an incommensurate modulated orthorhombic Pn2₁a(00??)000 structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic (c-AFM) ordering. Above T₁ = 543 K, the low temperature modulated Pn2₁a(00??)000 evolves monotonically into a fractionally growing Pnma structure up to T_N = 663 K. At 663 K, the low temperature c-AFM phase is suppressed concurrently with the switching of the former into a non-modulated Pn2₁a structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The Pn2₁a space group is obtained from the Pnma one through the  polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of a noteworthy linear spin-phonon coupling. In this regard, a magnetostructural coupling is observed below T_N, revealed by the relation between the weak ferromagnetism of the canted iron spins and the FeO₆ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature dependent magnetization and the magnetic peak intensity as well, yield a critical exponent (β) value of 0.38. The lower limit of the phase coexistence temperature T₁ = 543 K, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.